Don't call them "variants" in PackedPattern docs

https://unicode-org.github.io/icu4x/rustdoc/icu/datetime/provider/packed_pattern/struct.PackedPatterns.html#variants-1 We have internal (but documented) concepts of "variants" for PackedPattern, for distinguishing skeleta like `yM` and `yyyyM` and `GyM`. Unfortunately, `variant` also shows up in CLDR as `alt="variant"` on `dateFormatItem` (which these represent), which can be confusing. We should rename this concept.